This proposal consists of 3 projects. Project 1. will examine the role of external calcium in mediating calcium transients in the cytosol during oocyte maturation and fertilization in hydrozoan eggs which lack voltage dependent calcium channels. In these eggs calcium channels appear about the time of first cleavage. This project will also try to define the factors which are responsible for the initiation of calcium channel function at this time and map the distribution of the sites where the largest calcium transients can occur in eggs which are initiating their first cleavage. Project 2. will follow the clonal development of a specialized cell (the photocyte) in hydrozoans. Because of the small number of these cells in an organism, their fluorescence, and the small size of these organisms it is possible to follow each photocyte in a living organism on an individual basis. Three questions will be asked: 1) Under what circumstances can photocytes differentiate from non-photocyte cells? 2) Once a photocyte has formed what is its subsequent clonal history? and 3) What are the factors which regulate the de novo differentiation and proliferation of photocytes? Project 3. will examine the relationship between 2 mechanisms which specify dorsal-ventral axis formation in spiralians. One mechanism is associated with a cleavage pattern that generates a 4 cell stage in which one blastomere is larger than the others and has a unique developmental potential. The other mechanism is associated with a 4 cell stage in which the blastomeres are the same size and have the same developmental potential. Subsequently one blastomere becomes differentiated from the others as a consequence of an inductive interaction which randomly selects this blastomere. This project will alter the cleavage pattern so that one blastomere is larger than the others at the 4 cell stage in embryos that use this latter mechanism. The probability that this blastomere will have a developmental potential which is different from the others will be studied.